The present invention generally relates to methods of preparing microparticles, and more particularly to methods of preparing microparticles encapsulating biologically active agents, such as therapeutic agents.
Several methods for preparing microparticles are well known in the art. Examples of these processes include single and double emulsion solvent evaporation, spray drying, solvent extraction, phase separation, simple and complex coacervation, and interfacial polymerization. Methods developed for making microparticles for drug delivery are described in the literature, for example, in Doubrow, ed., xe2x80x9cMicrocapsules and Nanoparticles in Medicine and Pharmacyxe2x80x9d (CRC Press, Boca Raton 1992) and Benita, ed., xe2x80x9cMicroencapsulation: Methods and Industrial Applicationsxe2x80x9d (Marcel Dekker, Inc., New York 1996).
Emulsion-based processes usually begin with the preparation of two separate phases: a first phase, which generally consists of a dispersion or solution of an active agent in a solution of polymer dissolved in a first solvent, and a second phase, which generally consists of a solution of surfactant and a second solvent that is at least partially immiscible with the first solvent of the dispersed phase. After the first and second phases are prepared, they are combined using dynamic or static mixing to form an emulsion, in which microdroplets of the first phase are dispersed in the second, or continuous, phase. The microdroplets then are hardened to form polymeric microparticles that contain the active agent. The hardening step is carried out by removal of the first solvent from the microdroplets, generally by either an extraction or evaporation process.
Several U.S. patents describe solvent removal by extraction. For example, U.S. Pat. No. 5,643,605 to Cleland et al. discloses an encapsulation process in which the emulsion is transferred to a hardening bath (i.e. extraction medium) and gently mixed for about 1 to 24 hours to extract the polymer solvent. The long period of time required for extraction is undesirable, particularly if the process is to be operated continuously. Others have disclosed processes that compensate for the unfavorable thermodynamics (slow and incomplete extraction) by using a large excess of extraction medium. For example, U.S. Pat. No. 5,407,609 to Tice et al. teaches transferring the emulsion to a volume of extraction medium that is preferably ten or more times the volume required to dissolve all of the solvent in the microdroplets, so that greater than 20-30% of the solvent is immediately removed. U.S. Pat. No. 5,654,008 to Herbert et al. similarly discloses a process in which the volume of quench liquid, or extraction medium, should be on the order of ten times the saturated volume. The use of a large excess of extraction medium rapidly extracts a portion of the solvent from each microdroplet, creating a concentration gradient within each droplet and forming a polymer skin on the surface that advantageously traps active agent, but which disadvantageously slows extraction of the remaining solvent from the center portion of the microdroplet. Larger volumes of extraction medium also may increase process equipment and operating costs, as well as the costs associated with recycling or disposing of used extraction medium.
Evaporation is another approach known in the art for solvent removal. For example, U.S. Pat. No. 3,891,570 to Fukushima et al. and U.S. Pat. No. 4,384,975 to Fong teach solvent removal by evaporating an organic solvent from an emulsion, preferably under reduced pressure or vacuum. Solvent evaporation processes generally occur slowly enough such that the solvent/polymer composition remains uniform throughout each microdroplet during the evaporation step, such that a polymer skin is not created. For this same reason, however, the evaporation process is not favored for use with many active agents that partition into the continuous phase, resulting in significant loss of active agent into the continuous phase and/or the extraction medium, and consequently poor loading of active agent in the microparticle. Evaporation, however, would be highly desirable if such partitioning could be substantially avoided, since no extraction phase solvent and associated tanking and piping are required as in the extraction process.
One effort combining evaporation and extraction is disclosed in U.S. Pat. No. 4,389,330 to Tice et al. (xe2x80x9cTice ""330xe2x80x9d). Tice ""330 describes an emulsion-based method for making drug-loaded polymeric microspheres that uses a two-step solvent removal process: evaporation followed by extraction. The evaporation step is disclosed to be conducted by application of heat, reduced pressure, or a combination of both, and is purported to remove between 10 and 90% of the solvent. Tice ""330 also discloses that the extraction medium with dissolved solvent must be removed and replaced with fresh extraction medium, preferably on a continual basis. Consequently, the process requires either large volumes of extraction medium or an intermediate isolation of the microspheres combined with a change in the composition of the extraction medium.
Li et al., J. Controlled Release 37:188-214 (1995) (xe2x80x9cLi et al.xe2x80x9d) describes a model of a solvent removal process in which an emulsion of the dispersed phase is formed in a continuous phase devoid of dispersed phase solvent, extracting a portion of the dispersed phase solvent into the original volume for a brief period of time, and then further extracting the dispersed phase solvent by diluting the emulsion by continuous addition of continuous phase solvent. Evaporation of the dispersed phase solvent from the continuous phase/air interface is allowed to occur simultaneously with the extraction process, to maintain a driving force for extraction of the dispersed phase solvent into the continuous phase from the dispersed phase droplets. Uncontrolled evaporation of solvent from the open extraction vessel into the atmosphere is not practical or safe for production of greater than laboratory scale quantities, especially in a continuous process. In a closed vessel, the evaporation would rapidly cease as the solvent in solution equilibrated with the solvent vapor in the head space above the liquid surface. The Li et al. model also demonstrates that large extraction volumes are needed to operate the extraction process in relatively short time periods, as described in U.S. Pat. No. 5,407,609 to Tice et al., although the model predicts that skin formation and the glassy boundary can be achieved using total extraction solvent volumes that are less than the amount needed to dissolve all of the dispersed phase solvent.
It is therefore an object of this invention to provide methods for making microparticles efficiently encapsulating active agent.
It is another object of this invention to provide methods for making microparticles using a process that uses evaporation, controlled extraction, or a combination thereof to minimize the amount of extraction medium required in the process.
It is a further object of this invention to provide alternative methods of emulsion formation and solvent removal for use in processes of making microparticles.
It is another object of this invention to provide emulsion-based methods for making microparticles in an efficient batch or continuous process.
Processes for making polymeric microparticles, preferably including one or more active agents, have been developed. In a preferred embodiment, the process involves preparing (1) a dispersed phase containing an agent in a solution of polymer and a first solvent; (2) a continuous phase containing a surfactant, and a second solvent that is totally or partially immiscible with the first solvent; and (3) an extraction phase that is a nonsolvent for the polymer, a solvent for the continuous phase components, and a solvent for the first solvent, wherein the first solvent (or the first solvent component of greatest proportion if a mixture of solvents are used for the first solvent) has solubility in the extraction phase of between about 0.1% and 25% by weight. This limited solubility of the first solvent is important to maintain the stability of the emulsion as it is diluted. An emulsion of the dispersed phase in the continuous phase is prepared under appropriate mixing conditions, and the emulsion is then briefly mixed with a suitable quantity of extraction phase to concentrate the dispersed phase to induce skin formation at the interface of the dispersed and continuous phases. Remaining solvent is removed by an evaporation process step. The emulsification and solvent removal steps are preferably conducted in a continuous process. The brief extraction step prior to evaporation minimizes the loss of active agent from the microparticles, and reduces the required volume of extraction phase as compared to other extraction-based processes.
Alternate solvent removal processes are also provided. Some of these processes can be utilized in place of, or in combination with, known solvent removal processes and the preferred embodiment described above. In another preferred embodiment, solvent is removed using incremental, or cascade, extraction which involves introducing the extraction phase into the emulsion through a (temporal, not spatial) series of feed streams rather than a single feed stream, in order to slow and finely control the extraction of the solvent. In a further embodiment, solvent is removed by membrane separation, rather than direct mixing with the extraction phase. Other embodiments use cryogenic extraction, and two-phase solvent extraction, in which a single phase is used as the continuous phase and the extraction medium.
In preferred embodiments, the prepared continuous phase further includes dispersed phase solvent (i.e. first solvent), typically between about 10% and 100% of the amount needed to saturate the continuous phase.
In another preferred embodiment, a continuous process for making microparticles is provided, wherein the solvent is removed by evaporation, without the need for an extraction phase.
Alternate means of and processes for emulsion formation also are provided. For example, an emulsion lag tube, which is a small diameter tube of sufficiently high Reynolds number, is used to induce mixing in one embodiment of the process for making microparticles, rather than using conventional static mixers or agitators.